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Conveyer Belt Airport

I’m extremely pleased to bring you the Conveyer Belt Airport.

If you are unfamiliar with this concept let me quote the original question:

“A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?”

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I didn’t read the links, but I am fairly certain the answer is no, since planes require air passing at HIgh & Low pressure around the wing in order for it to move off the ground. The conveyor belt concept moves the ground, not the air required to lift the plane.

…Interestingly though, perhaps someone ould build an “air tunnel” that moves the air passed the plaes wings at a speed fast enough for it to lif off the ground…the problem then is that there is no momentum to carry the plane forward…so it would just sit there and hover until the air stopped.

Jeff: Oh, Jeff. What RSS reader-less rock have you been living under for the last, oh, 48 hours? It’s all in the links, man.

Chris: You guys always come in perfectly timed to shatter the tension with a little fit of laughter. I was one of many whose brain actually leaked out my eyes and onto my desk the other day. I’ve talked several people up about it and only really believe after seeing Matt’s link. Well done, as ever.

Steve: Oh, Steve. How I wish I had the time to read every link on every blog and still find enough time to sleep my four hours every night 😉

Ok, so after I found a moment I read the links, and the generally accepted conclsion is that the plane WOULD take off….I STILL disagree…and here is why. (Read the links for background as to what I am talking about) ;-P

The genaral argument is that the wheels on the plane don’t affect ANYTHING having to do with the thrust of the plane…which I agree. It is then said that the airflow of the engines/propellar moves the air providing the thrust to take off.

IMHO, this doesn’t work and here’s why….GRAVITY!. Gravity on the air around the plane & wings of the plane. Normally, the thrust produced by a jet engine moves the air froom the front of the engine, to the back of the engine. This movement of air is what in fact causes the plane to move forward, and this movement forwward is what causes the high/low pressure over the wings of an aircraft to provide lift. By placing a plane in a stationary position ( The riddle states the conveyer belt equalizes with the wheels), there is NO airflow over/under the wing. I believe the jet engine does NOT cause airflow over the wing, rather it causes propulsion through the air, and with the conveyer belt canceling out that propulsion, no lift can be applied!

….but I could, and have frequently been known to be wrong, especially in instances where I feel I am absolutley sure about it…like this one 😉 I have not research more than scanning the posts at the few blogs around, and don’t (sigh) have the time 🙂

haha… The idea is that no matter how fast the conveyer belt moves backwards, the plane will still end up moving forwards, because the propulsion isn’t from the wheels. The plane’s wheels exist to reduce friction to a negligable level; therefore it really doesn’t matter what the ground is doing…

….yes, I understand that idea, but still disagree. Propultion from the jets engines in my mind can NOT exist without forward movement. Otherwise, the engines are just spitting air from in front to the back of themseles…there is no air moving around the wings. So, in the case of his riddle, thrust is irrelavent…it is Lift that is crucial. No airflow (around the wings specifically), no lift.

..I’m pretty much still uncertain, but the merefact the majority of people feel the plane will take off makes me not want to think it will take off…a side effect of my rebelious youth. …that, and the fact that I think it won’t take off…no matter who disagrees with me. 😛

…I’m revising my position…I was wrong!…the plane will take off. Nobody said anything different…it’s just been soaking in my head for the past hour. The ligh tbulb went off ….I refuse to devote any more of my time to this topic, although I am sur eI will stil think about it running every scenario through my head for the next week…
last message,
-j

The confusion from this riddle is due to the fact that it’s worded to make you think that the plane is not allowed to move forward. So everyone says, “no. of course it can’t take off”.

But in reality, the plane is able to take off because it *is* able to move forward. Its jet engines are at full throttle and it *is* moving down the runway. Admittedly, its wheels are spinning like crazy, but it doesn’t matter, because the jet engines are using the air to push the plane forward. The jet engine is not related to the conveyor belt at all. Planes don’t use their wheels to propel themselves towards take-off. They use the air.

On another note… Here’s another riddle for you.

Scenario: There’s a two-story barn that is facing east to west. If the wind is blowing at 25 mph from the north, and a rooster lays an egg on the very peak of the roof of the barn… Which side of the barn will the egg roll down and fall off of? The left side, or the right side?

Now. I want you to give this riddle some genuine thought. Then, when you have the answer, scroll down:

Sure, a plane taking off on a real conveyor belt should take off fine (though I asked recreational pilot I work with and he said the tires would probably pop… but anyway).

The question to me came across as implying this is a ‘magic’ conveyor belt, that can in fact keep the plane in place relative to the ground (the idea that this isn’t how it would work in reality was irrelevant), in which case, of course it wouldn’t take off.

I don’t think the disagreement over the answer was a case of physics for most people, but imagination. Those who said right away that it can take off are boring! The rest of us chose more creative interpretations… 😉

My re-written question: “A jet plane with engines at full power is on a runway (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the planes forward speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?”

This problem can be understood quite simply by considering the example of a plane with frictionless bearings in the wheels. Examine the case when no thrust is applied and the conveyer moves at any speed. The plane will remain motionless with respect to the ground while the wheels spin at the conveyer speed. Now apply some thrust and the plane will start to move forward relative to the ground because in this case the speed of the conveyer (with frictionless bearings) can have no effect on the plane motion. As you increase thrust the plane will take off just like a normal plane. In fact from an observer on the ground the take-off will look perfectly normal. Now add some friction to the bearings and the only thing you will have to do is add a little more thrust to overcome this friction. Again from an observer on the ground the take-off looks exactly the same with the exception that the wheels seem to be spinning too fast. This is different from a car because the car must have locked (not frictionless) bearings (at least in one axel) in order to move.

The correct answer is that we don’t know. The runway might not be long enough for the plane to take off, the plane might not have enough fuel, or be overladen. It might be about to explode due to a malfunction. So all those people who argue either way are making assumptions they cannot know are true from the description of the problem.

However, it can take off from a purely physical perspective, there is nothing to stop it there.

the thrust must overcome the negative motion of said conveyer belt9ground0 lift is applyed only when air movementb is positive in overcoming negative motion the plane must move and air flow must move positive motion for lift.. no matter what speed of tires or ground said conveyer… you must obtain said lift.. sin cerily wiley cyotre geniiouuuse..

Look at it this way – What do the tires on a car move you with respect to? The road. Now what does the engine of a plane move you with respect to? The air!! So if the required air speed for a takeoff is 100mph – then the air must be moving across the wing at 100. if there is a head wind then the plane can use less thrust, whereas if there is a tail wind it will need more. Now the plane will move with respect to the air as before, the tires just make it easier for the plane to get across the runway. As stated above the tires will only be moving at twice the speed of the plane.

Right, for an aircraft to take off, it is propelled down the runway on wheels by engines. with me so far. The plane has to travel on the runway until it reaches v1 or v2 which is the speed at which the pilot says rotate, pulls back on the control column. you get the lift and it leaves the runway. Now, the conveyor runway is linked to the speed of the aircraft, which means if the plane is going 120 knots, the conveyor is doing the same in the opposite irection. A stationary aircraft, means no lift which means no take off. I thank you.

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[…] Well folks, do you remember the whole discussion about whether an airplane sitting on a conveyer belt runway would take off? It just so happens that someone from NASA posted a comment today at BrainFuel. […]